A spiral type gas separation membrane module in which a flat-film gas separation membrane is wound around a core tube generally has a configuration where a permeate-side spacer for forming a permeate gas channel is sandwiched between flat-film gas separation membranes, which are adhered to the core tube such that the permeate gas channel communicates with a hollow section in the core tube and are wound around the core tube together with a feed-side spacer for forming a feed gas channel.
Such a spiral type gas separation membrane module has a simple structure and can be easily manufactured, but gives a low concentration of a separated/recovered gas because of a lower separation efficiency in comparison with a hollow fiber type gas separation membrane module. It has not been, thus, necessarily preferentially used in practice.
Japanese laid-open patent publication No. 6-262026 has disclosed that in a spiral type gas separation membrane module for separating the air, a permeate gas channel is gradually made thicker from the initiation to the end of winding in order to prevent performance deterioration due to a pressure loss. However, this separation membrane module is provided for separating and recovering highly enriched nitrogen by feeding a pressurized air. The publication has not disclosed a method for separating and recovering oxygen-rich air from the air where a feed side is substantially at atmospheric pressure while a permeate side is under reduced pressure.
Separation and recovery of oxygen-rich air from the air using a gas separation membrane module inevitably involves discharge of nitrogen-rich air. In a method where a pressurized air is fed to a gas separation membrane module, oxygen-rich air at substantially atmospheric pressure is obtained in a permeate side while nitrogen-rich air under high pressure is obtained in a non-permeate side. That is, in the method of feeding pressurized air, nitrogen-rich air which is resultantly co-produced must be compressed as a feed air. Nitrogen is contained about four times as much as oxygen in the air. Therefore, in the light of an energy for compressing the nitrogen-containing feed air, the method of feeding pressurized air is energetically considerably inefficient and disadvantageous. Furthermore, for applying pressure to the feed side in the spiral type gas separation membrane module, the separation membrane module must be placed in a pressure-resistant vessel having nozzles of a feed side, a non-permeate side and a permeate side, or alternatively at least the outermost part of the separation membrane module must be of an adequately pressure-resistant structure or member. In addition, pressure regulating means such as a valve must be disposed at a given position in a gas channel for maintaining a feed-side pressure. Thus, the method requires extra components and/or structures.